Study: First Land Plants Appeared 500 Million Years Ago

According to a new study to be published in the Proceedings of the National Academy of Sciences, the first plants to colonize the Earth originated around 500 million years ago (Cambrian period) — 100 million years earlier than previously thought.

Rhynia gwynne-vaughanii, 400 million-year-old fossil plant stem from Aberdeenshire, Scotland. Image credit: Natural History Museum, London.

For the first four billion years of Earth’s history, continents would have been devoid of all life except microbes.

All of this changed with the origin of land plants from their pond scum relatives, greening the continents and creating habitats that animals would later invade. The timing of this episode has previously relied on the oldest fossil plants which are about 420 million years old.

The new study, led by University of Bristol’s Professor Philip Donoghue and Dr. Harald Schneider of the Natural History Museum, London, indicates that these events actually occurred a hundred million years earlier, changing perceptions of the evolution of the Earth’s biosphere.

Plants are major contributors to the chemical weathering of continental rocks, a key process in the carbon cycle that regulates Earth’s atmosphere and climate over millions of years.

The study authors used ‘molecular clock’ methodology, which combined evidence on the genetic differences between living species and fossil constraints on the age of their shared ancestors, to establish an evolutionary timescale that sees through the gaps in the fossil record.

“The global spread of plants and their adaptations to life on land, led to an increase in continental weathering rates that ultimately resulted in a dramatic decrease the levels of the ‘greenhouse gas’ carbon dioxide in the atmosphere and global cooling,” said co-author Dr. Jennifer Morris, from the University of Bristol.

“Previous attempts to model these changes in the atmosphere have accepted the plant fossil record at face value — our research shows that these fossil ages underestimate the origins of land plants, and so these models need to be revised.”

“The fossil record is too sparse and incomplete to be a reliable guide to date the origin of land plants,” said co-lead author Dr. Mark Puttick, also from the University of Bristol.

“Instead of relying on the fossil record alone, we used a ‘molecular clock’ approach to compare differences in the make-up of genes of living species — these relative genetic differences were then converted into ages by using the fossil ages as a loose framework.”

“Our results show the ancestor of land plants was alive in the middle Cambrian period, which was similar to the age for the first known terrestrial animals.”

One difficulty in the study is that the relationships between the earliest land plants are not known. Therefore, the scientists explored if different relationships changed the estimated origin time for land plants.

“We used different assumptions on the relationships between land plants and found this did not impact the age of the earliest land plants,” the authors said.

“Any future attempts to model atmospheric changes in deep-time must incorporate the full range of uncertainties we have used here.”

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J.L. Morris et al. Timescale of early land plant evolution: controlling for competing topologies and dating strategies on divergence time estimates. PNAS, published online February 19, 2018;